The present invention generally relates to a sensor for detecting anionic nutrients, and more particularly to a sensor that employs surface enhanced Raman spectroscopy (SERS ) for detecting very small concentrations of anions including toxic anions such as perchlorate, chromate, dichromate, and cyanide.
Detection and control of ionic pollutants (i.e., nitrates, sulfates, and phosphates) are important for the protection of the environment. When waste waters that contain ionic nutrients are discharged into surface waters, they can promote the unnatural growth of blue-green algae to the detriment of other plant and animal life. The decay of dead algae causes a reduction in the amount of dissolved oxygen available in the water. Eventually, the excess concentration of nutrients in the body of water results in the inability of the body to support any other life forms, a process called eutrophication. Besides eutrophication, ionic pollutants have been blamed for the increased incidences of red tide, blooms of toxic, single-celled organisms, such as dinoflagellates, that have caused die-offs of fish, dolphins, manatees, and other aquatic animals. Dinoflagellates are thought to be responsible for the human disease ciguatera. Symptoms of this disease include loss of coordination, slowed heartbeat, and diarrhea. Most people recover, but some victims die. Dinoflagellates are also blamed for paralytic shellfish poisoning.
Nitrates represents a potential human health hazard, and nitrate contamination is the most common reason for the shutdown of public water supply wells. When consumed, nitrates are converted to N-nitroso compounds in the human stomach. Some of these compounds are carcinogenic. It has been suggested that stomach cancer is associated with nitrate uptake. Furthermore, certain species of bacteria in humans can enzymatically reduce nitrate to nitrite. The toxic effects of nitrites include vasodilation, lowered blood pressure, and formation of methemoglobin, a non-oxygen carrying form of hemoglobin. In infants, methemoglobinemia is known as xe2x80x9cblue baby syndrome.xe2x80x9d
While ionic pollutants can enter the water supply by a number of natural means, the most significant contributions result from man made processes, such as fertilizers used in agriculture and effluents from sewage treatment plants. In order to protect the public health, a need exists for a sensor capable of monitoring ionic pollutants continuously, simultaneously, in real time, insitu, and with little or no sample preparation. Such a sensor needs to be able to differentiate ionic species, not suffer from interferences, be able to detect ppm concentrations of pollutants, and be reversible. Technologies which have been used in the past include colorimetry, UV-VIS absorption spectrometry, Raman spectrometry, electrochemical methods such as amperometry, or potentiometry using ion selective electrodes, and ion exchange chromatography. However, these approaches do not meet all the desired criteria of specificity, sensitivity, reversibility, real-time, etc.
In the 1970s, it was discovered that Raman scattering from molecules of an analyte of interest adsorbed on noble metals such as silver, copper, and gold when irradiated with optical energy can be enhanced by as much as 106 to 107 compared to merely irradiating the analyte.
This phenomenon is known as surface enhanced Raman spectroscopy (SERS). A SERS structure generally includes a silver, gold, or copper metal layer formed on a substrate and is used to detect the presence of an analyte by examining the emissions from the substrate when irradiated with optical excitation energy. SERS emissions, or spectra, have been used to detect and identify trace organic materials and as a detection method in gas chromatography, liquid chromatography, and thin layer chromatography. Electrochemical SERS and SERS of chemically modified surfaces have been used to detect aromatic compounds, chlorinated hydrocarbons, and other organic contaminants of environmental concern in the ppm range.
Therefore, a need exists for a robust sensor that can detect and identify trace amounts of anionic nutrients and which can overcome the aforementioned problems.
The present invention is directed to an integrated optical waveguide sensor system for detecting contaminants with a sensitivity of ppm and even ppb in some cases. The sensor includes: an optical waveguide having a monolithic and roughened metallic layer on which a self-assembled monolayer is formed; an optical energy source for generating an optical excitation signal to be coupled into the waveguide; and a spectrometer for detecting spectra of optical energy emitted from the optical waveguide. The waveguide facilitates multiple SERS responses resulting from interactions between the optical excitation signal and an analyte of interest that may be present on the surface of the self-assembled monolayer. Certain of the emitted spectra from the waveguide may be correlated to the presence of specific analytes in contact with the self-assembled monolayer.
These and other advantages of the invention will become more apparent upon review of the accompanying drawings and specification, including the claims.